1. Field of the Invention
The present invention relates to an image forming apparatus for color image formation by an electrophotographic method, such as a copying apparatus, a printer or a facsimile apparatus, and a method for use therein.
2. Description of Related Art
As an image forming apparatus for forming a color image, there is commercialized an apparatus capable of forming a color image for example by transferring, on a transfer material such as paper supported on a transfer drum (or transfer film), in succession toner images of respective colors formed on a photosensitive drum serving as an image bearing member.
In such image forming apparatus, an electrostatic latent image formed on the photosensitive drum in response to an input image signal is developed with toner of a first color (for example cyan color) to form a toner image, which is transferred onto a transfer material such as paper supported on a transfer drum (transfer film). This transfer step is executed similarly for the toners of three other colors, namely magenta, yellow and black, and a color image can be obtained by superposed transfers of the toner images of four colors in succession on the transfer material.
In an image forming apparatus of electrophotographic type utilizing a recent digital image signal, the latent image is formed by an assembly of dots of a predetermined potential on the surface of a latent image bearing member, or so-called photosensitive member, and a solid image area, a halftone area and a line image area are represented by a change in the density of the dots. In such method, however, it is difficult to deposit the toner particles faithfully on the dot and the toner particles tend to overflow from the dot, thereby leading to a drawback that the gradation of the toner image cannot be obtained corresponding to the ratio of the dot density in a black image area and a white image area in the digital latent image. Also in case the resolution is increased by reducing the dot size for the purpose of improving the image quality, the latent image formed by such small dots becomes more difficult to reproduce, thus often resulting in an image with insufficient sharpness, poor in resolution and in gradation particularly in a highlight area. Also an irregular deviation of the dots gives an impression of granularity, thus constituting a factor leading to a deterioration of the image quality in the highlight area.
Such irregularity, not existent in ink jet recording technology or printing technology, is considered as a major drawback in that it is an unpredictable instability factor in the image quality and is a low-frequency noise macroscopically generated by a random distribution of small toner particles of a size of 5 to 10 μm along the dot contour. Observation of an electrophotographic image under a magnifier reveals that a dot in the electrophotography does not have a smooth contour as in an ink jet recorded image but is formed by a random distribution of small toner particles of a size of 5 to 10 μm along the dot contour. Also the dots fluctuate in the density and in the dot size, and are not circular but distorted in shape, so that none of them looks alike. These parameters show almost random fluctuation, with a considerable low-frequency component, which leads to a noise perceivable to the human eyes.
Such noise is made more conspicuous by a difference between the toner density and the density of the paper. Particularly in comparison for example with an ink jet recorded image, there is a significant influence of an optical dot grain resulting from distribution of numberless small toner particles.
A main cause of the aforementioned phenomena lies in a fact that the electrophotography utilizes small toner particles for forming a dot. There are also various associate factors, including an unsharpening of dot data in the steps of latent image formation, image development and image transfer in the electrophotographic process, an irregular toner scattering resulting from physical properties (electrical resistance and surface roughness) of the copying paper, and a phenomenon, to be explained in the following, caused by an adhesion force in the development process.
While a strong adhesion force (mainly a mirror force of the toner to a developer carrying member) is exerted between the toner and a developing sleeve in case of a one-component developer or between the toner and a carrier in case of a two-component developer, the distribution of the charge amount on the toner is not uniform. Therefore, when the toner is peeled off by a developing bias and is made to fly toward the photosensitive drum, the image formation takes place in an unstable manner as the toner flies more easily in a certain location while more difficultly in another location, thus resulting in an unevenness in the dot formation.
On the other hand, in the ink jet system, a hyperchromic-hypochromic ink process as disclosed in Japanese Patent Application Laid-Open No. S58-39468 does not encounter the aforementioned drawbacks in the electrophotographic process, since the ink jet system itself is simple and since the exclusive paper supporting the current high ink jet image quality has an excellent ability.
Therefore, with respect to the improvement of granularity which is an effect of hyperchromic-hypochromic inks employed for example in the ink jet system, it is found that the hypochromic toner is far more effective in the electrophotography than in the ink jet process, particularly on the apparent low-frequency noise resulting from the aforementioned phenomena of “fluctuation in the toner density forming the dot”, “fluctuation in the dot area” and “fluctuation in the dot shape” in the electrophography.
In addition, it is found that the introduction of the hypochromic toner into the electrophotography brings about a revolutionary improvement based on a fact that the optical dot gain which is not a problem in the ink jet system constitutes a significant obstacle in attaining a high image quality in the electrophotography utilizing numberless small toner particles.
For attaining these improvements, there have been proposed methods of forming an image by using a pale-colored toner (hypochromic-toner) in a highlight area and a dense-colored toner (hyperchromic toner) in a solid image area. Japanese Patent Application Laid-Open Nos. H11-84764 and 2000-305339 propose image forming method by combining plural toners of respectively different densities. Japanese Patent Application Laid-Open No. 2000-347476 proposes an image forming apparatus utilizing a combination of a hyperchromic toner and a hypochromic toner, having a maximum reflective density equal to or less than a half of the maximum reflective density of the hyperchromic toner. Japanese Patent Application Laid-Open No. 2000-231279 proposes an image forming apparatus utilizing a combination of a hyperchromic toner, having an image density of 1.0 or higher at a toner amount of 0.5 mg/cm2 on the transfer material, and a hypochromic toner having an image density less than 1.0. Also Japanese Patent Application Laid-Open No. 2001-290319 proposes an image forming apparatus utilizing a combination of a hyperchromic toner and a hypochromic toner, having an inclination ratio of recording density within a-range of 0.2 to 0.5.
A high-resolution binary recording of a high level, stably realizable in the ink jet recording process relying on an exclusive paper, cannot be realized on a plain paper which is principally utilized in the electrophotographic apparatus. For this reason, density gradation has been improved for example by utilizing a low-resolution screen as employed in the printing technology. Though the use of a hypochromic toner in a low density area achieves an improvement in coarse and granular impression specific to the electrophotography, it is found, in designing a system particularly intended for photographic image quality, that an image giving emphasis on the smoothness of the highlight area is often selected as an image for using hyperchromic and hypochromic toners, and, in such case, streaks caused by a screen used for the hypochromic toner are very easily noticeable and a photographic texture is difficult to attain. Also in the high density side, an output condition requiring a high resolution is usually selected because such side is often used for the output of characters, but, in a photograph or the like usually selected as an image for using hyperchromic and hypochromic toners, the output image desired for the high density area does not necessitate such a high resolution as required in the character image but factors such as gradation levels, density difference and color difference are more important, and the image output is more stabilized in fact with a screen of a lower line numbers.